Exodus 13:8-10

And thou shalt tell thy son in that day, saying, It is because of that which Jehovah did for me when I came forth out of Egypt.

And it shall be for a sign unto thee upon thy hand, and for a memorial between thine eyes, that the law of Jehovah may be in thy mouth: for with a strong hand hath Jehovah brought thee out of Egypt.

Thou shalt therefore keep this ordinance in its season from year to year.